The present invention relates to an apparatus used in conjunction with a wastewater treatment process involving the aeration and biological decomposition of organic waste matter. More particularly, the present invention pertains to clarification devices used with aeration devices having a channelized, circuitous flow of wastewater.
Channelized aeration devices normally involve an oval ditch or tank containing a level of wastewater, a means to induce a flow in the tank and a source of aeration. Typical methods of inducing flow and/or aeration include mechanical devices such as paddles or impellers, as well as forced air jets. As the wastewater circulates in the tank, biological organisms digest unwanted organic substances. The aeration of the water is designed to promote the survival of the biota and enhance their activity. Typically the water circulates in the tank for a speciried period of time, or until a desired reduction in organic wastes is achieved.
An alternate form of channelized aeration device involves a vertically oriented channelized reactor or vertical looped reactor (VLR) of the type disclosed in U.S. Pat. No. 3,703,462 to Smith. In that device, an elongate tank is provided with a horizontal baffle joined to the side walls of the tank and is almost coextensive with the end walls. The tank is designed so that a cyclical over and under flow of water is induced by a combination of mechanical and pressurized means.
It has been customary to transfer treated effluent from channelized aeration tanks to a separate clarifier tank for final solids separation. Clarifier tans often comprise a relatively quiescent containment area where solid materials may settle out and floating wastes are skimmed from the surface of the water. Both settled and floating materials are collected and disposed of, either by recirculating them to the aeration tank, or by conveying them to anaerobic digesters.
Rising capital and maintenance costs have created a demand for treatment devices having a minimum of moving parts and consuming relatively small amounts of power. To answer this demand for greater economy, some manufacturers have designed inchannel clarifiers which may be inserted directly into the flow of the aeration channel. These in-channel clarifiers have a minimum of moving parts, and are normally comprised of a tank and a combination of weirs and baffles which divert water from the main flow into a quiescent zone where solids separation can occur.
One such device is an intrachannel clarifier which consists of an open-bottomed tank positioned in a flow channel so that circulating wastewater flows underneath the open-bottomed tank. A series of angled baffles diverts a portion of the flow into the quiescent settling area which is defined by the walls of the open-bottomed tank. Solids settle out and are returned to the flow by the same baffles through which solids entered the quiescent zone. A submerged orifice discharge pipe is provided for removal of clarified effluent.
This and other intrachannel clarifiers presently on the market have three main disadvantages. First, the placement of the clarifier tank in the flow channel creates an impediment to the free flow of wastewater. Consequently, more energy is required to maintain a desired flow velocity, and the retention time of water in the tank must be increased to compensate for the reduction in tank capacity caused by the presence of the clarifier.
Second, intrachannel clarifiers promote the stagnation of sludge and resulting undesirable side effects. This stagnation results from the fact that the same baffled passageways are used for the diversion of wastewater into the clarifier as well as for the escape of settled solids back into the channelized flow for further biodegradation.
Stagnation occurs when lighter weight solids settling from water closer to the surface of the clarifier tank are intercepted by the incoming flow containing a greater proportion of heavier solids. These heavier solids tend to settle out first, and, when the effect of these heavier solids is combined with that caused by the velocity of the clarifier in-flow, the lighter solids are left with no means of reaching the channelized flow. Thus, the intrachannel clarifier fosters a "last in, first out" flow pattern.
A further consequence of the stagnated solids is a significant increase in cell breakup or lysing. This process occurs when biota attached to suspended organic matter is deprived of sufficient oxygen as a result of being retained in the clarifier tank for an excessive length of time, usually in the realm of two to three days. As the biota die off through suffocation, their cell structure breaks down, releasing organic compounds into the wastewater. Thus, materials which would normally settle out as a part of the suspended solid matter are released into the wastewater where they dissolve, becoming much more difficult to remove. A further disadvantage of lysing is an increase in the biochemical oxygen demand (BOD) of the wastewater. Thus, cell lysis impedes water treatment, for healthy organisms are necessary for optimum BOD removal in the clarified effluent.
Third, these intrachannel clarifiers have no positive means of removing scum unless costly mechanized skimming devices are incorporated into such tanks.
Consequently, there is a definite need for a clarifier which is compatible with channelized flow aeration tanks, which provides for proper circulation of wastewater therethrough, which minimizes solids stagnation and cell lysis, and which positively removes scum.
It is therefore an object of the present invention to provide a clarifier having no moving parts which is designed to operate within a channelized flow aeration tank.
It is another object of the present invention to provide a side flow clarifier which provides for proper solids cycling and removal.
It is a further object of the present invention to provide a low cost, side flow clarifier which does not impede the circulating flow of wastewater in the aeration channel.
It is a still further object of the present invention to provide a low cost, nonmechanized means of positively removing floating scum.